In a SiC (silicon carbide) semiconductor device, it is effective to increase a channel density for flowing a large current. In view of this point, a trench gate type MOSFET is used for a silicon transistor. The SiC semiconductor device may have a trench gate structure. Thus, a vertical type MOSFET having a trench gate structure made of SiC is proposed in JP-A-2008-177538, JP-A-2008-294210 and JP-A-2009-289987.
However, when the vertical type MOSFET having the trench gate structure is manufactured, a concavity and a convexity may be formed on a sidewall and/or a bottom of a trench. Thus, a gate insulation film disposed on the sidewall and the bottom of the trench may have low insulation withstand voltage and low lifetime. Since the breakdown electric field intensity of the SiC semiconductor device is ten times larger than the silicon semiconductor device, the SiC semiconductor device is used under a condition that a voltage almost ten times larger than that of the silicon semiconductor device is applied to the SiC semiconductor device. Accordingly, the electric field having the intensity ten times larger than that in the gate insulation film of the silicon semiconductor device is applied to the gate insulation film of the SiC semiconductor device. Thus, the insulation withstand voltage and the lifetime of the gate insulation film become important.